The present invention relates generally to apparatus for detecting the presence of a specially constructed target, and more specifically to a novel high contrast target discrimination system, particularly suitable for automatic guided vehicles, that can be quickly and remotely switched to choose from among a plurality of targets or guidepaths.
Automatic guided vehicles (AGVs) are unattended industrial vehicles for automatically moving material between and within warehouses, offices and factories. The guidance systems for AGVs need to be very accurate AGV guidance systems generally use floor-mounted guidepaths that the guidance system detects and follows. The prior art includes such guidepaths as buried wires, called guide wires, that are detected by the presence of a magnetic field around the wires that results from running an electric current through the wires; and painted or taped lines, called guidelines, that are detected by the reflection of a bright light source from the lines. An alternative to floor-mounted guidepaths are retroreflective targets mounted aboveground throughout a building that an optical guidance system "sees" by reflecting light off the target and then computing its location.
A disadvantage of guide wire, or wire-guided, AGV guidance systems is that they generally are not flexible enough to deal with the constantly changing work flow of modern manufacturing and warehousing environments. Each change requires cutting costly new slots into the floor for the wires. Wire-guided systems can, however, be wired like a model railroad set to provide remotely selectable multiple guidepaths.
Guideline, or other reflected light, systems require a high contrast between background illumination and the light reflected from the guidelines or target. A useful reflected light guidepath system is disclosed in U.S. Pat. No. 3,935,922 to Cooper et al. The Cooper et al patent discloses directing ultraviolet light onto a line containing fluorescent material to cause the guideline to fluoresce and re-emit visible light which is detected by photocells. Such fluorescent systems inherently achieve some increased contrast because the fluorescent material emits light at a particular wavelength for which the detection system can filter the incoming light to better discriminate. This also eliminates the problem with some visible light reflecting systems that can be fooled by light reflected back from other shiny surfaces. The contrast is additionally increased by modulating the output ultraviolet light at a preselected frequency so that the fluorescently re-emitted visible light is similarly modulated and the detector can selectively filter out background illumination of the same wavelength, but not the same modulation frequency.
Other prior art reflected light systems increase contrast by using polarized emitted and reflected light. These systems typically direct polarized light at a target that reflects the light without changing the polarization. Light reflected from areas other than the target, or specially marked portions of the target, are reflected with a generally more random polarization so that less light from non-target areas will pass through a polarizing filter placed in front of a detector than will the more highly polarized light from the target. The prior art also includes using bulky and awkward corner reflectors as targets to rotate the polarization of the reflected light by 90.degree. and then detecting for the thus rotated polarized light.
Unfortunately, like buried wire systems, optical guidepath guidance systems suffer from a number of disadvantages. For example, guidelines frequently, especially in hostile environments, become covered with dirt and are damaged by foot and machinery traffic. Also, fluorescent systems cannot practically be used with retroreflective targets because projecting a beam of ultraviolet light more than ten feet generally requires an ultraviolet laser, which is both expensive and dangerous to personnel in the area. Moreover, the re-emitted visible light from the fluorescent target will generally be too weak to be detected at practical distances.
The increased flexibility offered by the lower cost of placing new optical guidelines over installing new buried guide wires has made optical guidepaths the more common choice in industry. Unfortunately, this flexibility so far extends only to the ease of placing new guidelines. The prior art has not found a means for adding to guideline systems the desirable remote and quick switching among different guidepaths that is found in some guide wire systems. The prior art does include means for semi-permanent routing changes to, for example, branch lines by painting dark cross-bars, also used to indicate stops, across the guidepath before the beginning of a branch line to signal the AGV to make a turn onto the branch line. Typically, such systems are designed so that the AGV will always make a right turn at a junction, except where a particular sequence of crossbars are present before the junction. In working environments where repeated changes are desirable, having to remove and/or repaint crossbars each time a change is desired is awkward and inconvenient. Moreover, the changes cannot, of course, be performed remotely.
Thus it is seen that there is a need for optical guideline AGV guidance systems that can be quickly and remotely switched among different guidelines.
It is, therefore, a principal object of the present invention to provide a high contrast target discrimination system, particularly suitable for automatic guided vehicles, that can be quickly and remotely switched to choose from among a plurality of targets or guidepaths.
It is another object of the present invention to provide an improvement to polarization methods used in reflected light target discrimination systems for AGVs.
It is a feature of the present invention that its capability to remotely switch among various guidepaths can also be used to remotely stop and start AGVs.
It is another feature of the present invention that conventional controllers can be easily adapted for use with the invention.
It is a further feature of the present invention that it is particularly suitable for use with retroreflective targets.
It is an advantage of the present invention that it provides the high contrast of fluorescent systems without the accompanying disadvantage of short range.
It is another advantage of the present invention that its implementation will be straightforward and uncomplicated.
These and other objects, features and advantages of the present invention will become apparent as the description of certain representative embodiments proceeds.